An assembly attached to the rear of a car (or truck) that is designed for drag racing. The purpose is to prevent the front wheels from lifting too far off the ground, thus avoiding unwanted wind resistance and potentially the front wheels lifing so far off the ground that the whole car flips over on its roof.
Typically, a wheelie bar looks like an "A" with 2 wheels attached to the top of the "A" with the base being attached to the rear frame of the vehicle along with a spring.
Basically, if you happen to see a car that NEEDS one of these out on the road, dont drag race it. You WILL lose. Cars that need this usually run 10 seconds or less in the quarter mile.

I just bought a wheelie bar dude! Finally I can floor it without having to worry about my ride flipping over! I can't wait to see my times at the track!